But there is a less well-known side of nuclear power: It requires great amounts of cool water to keep reactors operating at safe temperatures. That is worrying if the rivers and reservoirs which many power plants rely on for water are hot or depleted because of steadily rising air temperatures.

If temperatures soar above average this summer - let alone steadily increase in years to come, as many scientists predict - many nuclear plants could face a dilemma: Either cut output or break environmental rules, in either case hurting their reputation with customers and the public.

For details on why this is not an insurmountable problem, click here and here for posts from our archives. Here's an excerpt from one of those posts by my friend Lisa Stiles:

It doesn't matter if you're burning uranium, coal, oil, or cow dung, anything that uses a steam cycle has the potential problem of exceeding discharge limits if temperatures are excessively warm. Since only about 1/3 of the heat is usable to turn a turbine, the waste heat has to go somewhere. To not have this problem you can:

--Not make the environmental regulations overly conservative--Build a bigger heat sink--Build a smaller plant--Invent a thermodynamic cycle better than the ones the world's best minds have come up with in the past two centuries or so (and be sure to include my name on the patent).

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I agree with Lisa's explanation all the way up to the point where she says "Invent a thermodynamic cycle better than the ones the world's best minds have come up with in the past two centuries or so (and be sure to include my name on the patent)."

Smaller, more distributed plants can help by effectively providing a larger heat sink per unit waste heat produced. Larger heat sinks work, and certainly changing the rules to allow hotter discharges work (it is certainly worth investigating, but there is a real risk to plant and animal life that must be understood.)

With regard to "better" thermodynamic cycles, there are others available that do not have as much of a problem in exceeding discharge limits. Combined cycle gas turbines and cogeneration plants that make better use of their plant's input heat have been around for many years. Since they obtain thermal efficiencies approaching 60% instead of the 33% common to the simple Rankine cycle used by conventional light water reactors, they have less waste to get rid of.

There is a distinct possibility that nuclear combined cycle or cogeneration plants will be available in the not too distant future - the PBMR out of South Africa may be a suitable base plant for such a development.

The issue with the French plants is that they use direct cooling with river water. The simple solution to this problem is to use a cooling tower, then there is no limitation from temperature increases to local bodies of water (rivers or lakes). This is the solution that is being implemented for the new ESBWR at North Anna, where there were concerns about too much additional heating of Lake Anna (which is an artificial lake built to cool the original reactors). Heat rejection is no problem if one uses cooling towers.

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